It is known to include elongate elements such as threads or ribbons of material such as plastics film, metal foil, metallised plastics and metal wire in the thickness of paper to render imitation of documents produced from the paper more difficult. To increase the security given by the included element, it has been proposed to endow the element itself with one or more verifiable properties over and above its presence or absence. Such properties include magnetic properties, electrical conductivity, the ability to absorb X-ray strongly and fluorescence.
Also it has been proposed in British Specification No. 1552853 to use a dichroic filter material as the elongate element and to provide apertures in the sheet so that the dichroic material is exposed on both sides at the same location and viewable by reflected or transmitted light. The dichroic material will have a different appearance when viewed in these different ways.
The techniques proposed for providing the said apertures all however suffer from the disadvantage that they are applied after the sheet containing the security element has been made.